As integrated circuit geometries become increasingly smaller, an abnormal mode of operation, sometimes termed "avalanche breakdown" or "punchthrough" occurs. When punch-through occurs, the drain current is, essentially, no longer under the control of the gate.
For example, in an n-channel transistor built upon a p-type substrate, the source and drain regions are both n-type. Essentially, one observes two back-to-back diodes when the transistor is off. If the channel is short enough and the drain voltage high enough, the depletion region of the n-p diode associated with the drain will eventually touch the source. Uncontrollable transistor leakage results.
Designers have sought to minimize punch-through effects by implanting appropriate dopant species into the channel region. For example, in an n-channel device (i.e., a device with n-type source and drains) a p-type dopant species is often implanted into the channel region adjacent the source and drain. The implant, termed a punch-through control implant, helps to prevent the undesirable phenomena described above. Various approaches have been employed in the past to perform punch-through control implanting. One approach involves the formation of the gate with flanking spacers. The gate, together with the source and drain, is subsequently salicided. Then the spacers flanking the gate are removed. A punch-through control implant species is directed toward the channel region through that portion of the substrate left exposed by the removed spacer. The silicide over the gate, source and drain regions stops or retards the dopant species, thereby helping to insure that the punch-through control implant is properly positioned near the channel.
However, the above approach presents a variety of practical problems in manufacture. When p-type dopants are employed, the dopant tends to degrade the silicide. Furthermore, the thickness of the silicide is critical to proper absorption of the p-type dopant over the gate, source and drain. (Thicker silicides are desirable for stopping the p-type dopant from penetration. However, thicker silicides may induce junction leakage.) Finally, it is difficult to selectively and completely strip the spacer which flanks the gate.